1. Field of the Invention
The present invention relates to an electronic bell sound generating circuit for a timepiece and more particularly to a circuit network having a simpler structure than conventional systems and which is capable of generating an electronically simulated sound bearing a close resemblance to a bar bell sound or bell sound generated by a motor driven bell-striking mechanism.
2. Prior Art
Electronic sounds which have been used conventionally as time signals for timepieces are inferior in sound quality to natural bell sounds or bar bell sounds and they frequently cause some unpleasantness to the senses of the listeners. Therefore, in an effort to bring these electronic sounds closer to natural bell sounds and bar bell sounds, attempts have been made to mix a plurality of various frequencies or to use systems designed to graudally attenuate the volume of sound.
As a method for attenuating the sound volume, conventionally the charge and discharge of condensers and resistors provided outside of the timepiece circuits have been used. However, in recent years, in order to obtain a more accurate and complicated attenuated sound, resistance control circuits for stepwisely decaying the quality of current to the sound generating circuit that functions to provide the bell sounds have been employed.
The resistance control circuit system described above has been constructed by connecting a plurality of circuits in parallel. These respective circuits are provided with resistances and transistors serving as electronic switches which are connected in series. Accordingly, the system operates in the following manner. After the hour tone of the timepiece has been generated, at each specified time interval, the transistors are turned on or off in sequence. In this way, combined resistance value of the resistors connected in series to the transistors is gradually increased. This in turn causes a graduate decrease in value of current to the sound generating circuit. As a result, an alarm tone with a gradually decreasing sound volume is produced.
This above described system enables an attenuating sound with an optional pattern to be produced by controlling and varying the speed for the sequential turning on or turning off of the transistors as well as setting the resistance value of the resisters. As a result, the sound thus formed can be further approximated to the natural bell sound or bar bell sound.
However, this system has certain disadvantages. That is, in this system, when the timepiece circuit is made into an integrated circuit, although transistors used as the electronic switch can be built into the integrated circuit, the plurality of resistors connected in series to the transistors must be provided outside of the integrated circuit. Consequently, the number of IC pins, the manufacturing process and part required for making up the system increases. As a further result, the cost for producing this system is also increased.